If you are including an unformatted block of text into a single merge field
and expect to format it during the merge, you are going to be struggling. If
the text is formatted, then a \*mergeformat switch on the field should bring
over the formatting. The only other way would be to bookmark the separate
bits of text in the source document and use includetext fields to import the
bookmarked parts separately (without the mergeformat switch), whereupon you
can use normal style formatting in the merge document to ensure they appear
as intended.