It is indeed extremely annoying that it doesn't seem possible to simply add one or more styles from your current document to the default template.
However, what is possible is to copy styles from one open document to another open document. In some situations, that may be sufficient as a workaround.
If you have a document open in Writer or in Calc, with the styles you need:
- assign the wanted style(s) to some empty lines or empty cells
- copy the empty lines or cells
- open the other document in which you want to have these custom styles
- paste the (empty) lines or cells into your other document.
Now, the custom styles from the first document will also be available in the second document.