STEP 1:
On some other online system first download all packages and requirements. Preferably the system will be the same distro and version as the offline host.
To download all required packages you could do something like this:
$ sudo apt update
$ mkdir x11vnc; cd $_
$ for i in $(apt-cache depends --recurse --no-suggests --no-conflicts --no-recommends --no-breaks --no-replaces --no-enhances x11vnc | grep "^\w"); do apt download $i; done
The above will download all required packages into the x11vnc directory.
STEP 2:
Copy the entire x11vnc to your offline system. On my test run this was exactly:
$ du -sh x11vnc
21M x11vnc
Just for an illustration this is what the content of the x11vnc directory might look like:
~/x11vnc$ ls
cdebconf_0.250ubuntu2_amd64.deb libc6_2.30-0ubuntu3_amd64.deb libgpg-error0_1.36-7ubuntu1_amd64.deb libselinux1_3.0-1_amd64.deb libxcb1_1.13.1-3build1_amd64.deb perl-base_5.30.0-9_amd64.deb
coreutils_8.30-3ubuntu2_amd64.deb libcap2_1%3a2.27-1_amd64.deb libhogweed5_3.5.1+really3.5.1-2_amd64.deb libslang2_2.3.2-4_amd64.deb libxdamage1_1%3a1.1.5-1_amd64.deb sensible-utils_0.0.12+nmu1_all.deb
debconf_1.5.73_all.deb libdb5.3_5.3.28+dfsg1-0.6ubuntu1_amd64.deb libidn2-0_2.2.0-2_amd64.deb libssl1.1_1.1.1d-2ubuntu3_amd64.deb libxdmcp6_1%3a1.1.3-0ubuntu1_amd64.deb tar_1.30+dfsg-6_amd64.deb
dpkg_1.19.7ubuntu2_amd64.deb libdbus-1-3_1.12.16-2ubuntu2_amd64.deb libjpeg8_8c-2ubuntu8_amd64.deb libsystemd0_244.1-0ubuntu2_amd64.deb libxext6_2%3a1.3.4-0ubuntu1_amd64.deb tcl8.6_8.6.10+dfsg-1_amd64.deb
fontconfig-config_2.13.1-2ubuntu2_all.deb libdebian-installer4_0.110ubuntu3_amd64.deb libjpeg-turbo8_2.0.3-0ubuntu1_amd64.deb libtasn1-6_4.15.0-2_amd64.deb libxfixes3_1%3a5.0.3-1_amd64.deb tcl_8.6.9+1_amd64.deb
fonts-dejavu-core_2.37-1_all.deb libelogind0_241.3-1+debian2_amd64.deb liblz4-1_1.9.2-2_amd64.deb libtcl8.6_8.6.10+dfsg-1_amd64.deb libxft2_2.3.3-0ubuntu1_amd64.deb tk8.6_8.6.10-1_amd64.deb
fonts-liberation_1%3a1.07.4-10_all.deb libexpat1_2.2.9-1_amd64.deb liblzma5_5.2.4-1_amd64.deb libtextwrap1_0.1-14.2_amd64.deb libxi6_2%3a1.7.10-0ubuntu1_amd64.deb tk_8.6.9+1_amd64.deb
gcc-10-base_10-20200202-1ubuntu1_amd64.deb libffi7_3.3-3_amd64.deb liblzo2-2_2.10-2_amd64.deb libtk8.6_8.6.10-1_amd64.deb libxinerama1_2%3a1.1.4-2_amd64.deb ttf-bitstream-vera_1.10-8_all.deb
libacl1_2.2.53-5ubuntu1_amd64.deb libfontconfig1_2.13.1-2ubuntu2_amd64.deb libnettle7_3.5.1+really3.5.1-2_amd64.deb libunistring2_0.9.10-2_amd64.deb libxrandr2_2%3a1.5.2-0ubuntu1_amd64.deb tzdata_2019c-3_all.deb
libattr1_1%3a2.4.48-5_amd64.deb libfreetype6_2.10.1-2_amd64.deb libnewt0.52_0.52.21-3ubuntu1_amd64.deb libuuid1_2.34-0.1ubuntu5_amd64.deb libxrender1_1%3a0.9.10-1_amd64.deb ucf_3.0038+nmu1_all.deb
libavahi-client3_0.7-4ubuntu6_amd64.deb libgcc1_1%3a10-20200202-1ubuntu1_amd64.deb libp11-kit0_0.23.19-2_amd64.deb libvncclient1_0.9.12+dfsg-8_amd64.deb libxss1_1%3a1.2.3-1_amd64.deb x11-common_1%3a7.7+19ubuntu13_all.deb
libavahi-common3_0.7-4ubuntu6_amd64.deb libgcc-s1_10-20200202-1ubuntu1_amd64.deb libpcre2-8-0_10.34-7_amd64.deb libvncserver1_0.9.12+dfsg-8_amd64.deb libxtst6_2%3a1.2.3-1_amd64.deb x11vnc_0.9.16-3_amd64.deb
libavahi-common-data_0.7-4ubuntu6_amd64.deb libgcrypt20_1.8.5-3ubuntu1_amd64.deb libpng16-16_1.6.37-1_amd64.deb libx11-6_2%3a1.6.8-1_amd64.deb libzstd1_1.4.4+dfsg-1_amd64.deb zlib1g_1%3a1.2.11.dfsg-1ubuntu3_amd64.deb
libbsd0_0.10.0-1_amd64.deb libgmp10_2%3a6.1.2+dfsg-4_amd64.deb libsasl2-2_2.1.27+dfsg-2_amd64.deb libx11-data_2%3a1.6.8-1_all.deb lsb-base_11.1.0ubuntu2_all.deb
libbz2-1.0_1.0.8-2_amd64.deb libgnutls30_3.6.11.1-2ubuntu2_amd64.deb libsasl2-modules-db_2.1.27+dfsg-2_amd64.deb libxau6_1%3a1.0.9-0ubuntu1_amd64.deb openssl_1.1.1d-2ubuntu3_amd64.deb
STEP 3:
On the offline system enter the x11vnc directory you just copied and execute:
$ sudo dpkg -i *
This should install all packages including dependencies as well as the x11vnc package. Note: you might encounter some errors as some of the packages might already be installed etc.