You said it: the value of a field is a "single atomic sequence". All you can do is apply a single character style to it to change its aspect from the surrounding paragraph.
I experimented with a pasted DDE link where the source text had several character styles applied to it (and also contained various fields). The field value is a string of characters stripped of any stylistic variation (similar to unformatted text).
The only way I found to keep original formatting was to Insert>Section with Link option (without DDE).
I emphasise the word keep to mean you have a mirror of the original file. If you want to be able to change formatting, you must "unprotect" the section, but next time you Tools>Update>Update All, Writer retrieves the content of the file and all your formatting changes are lost.
I could not succeed in using the DDE link of a section (the command I entered was ineffective and I didn't search why).
To show the community your question has been answered, click the ✓ next to the correct answer, and "upvote" by clicking on the ^ arrow of any helpful answers. These are the mechanisms for communicating the quality of the Q&A on this site. Thanks!
In case you need clarification, edit your question (not an answer which is reserved for solutions) or comment the relevant answer.
