What do you mean by appending/merging the fields? Do you mean that both the NEF and the JPG contain information in the fields and you want to have the result be both values added? E.g. the caption in the JPG contains "JPG caption", the NEF contains "NEF caption", the result should then be "JPG CaptionNEF Caption" or something similar?
This is currently not possible (but read on for potential work-around) and also introduces lots of questions like: what field should go first, what separator should be used (space, newline, nothing?), etc. This would be very complex, if not undoable...
However, you may try to see if the following would work:
1. Set up an IPTC template that copies fields that you want "merged/appended" to another field (e.g., caption to headline: Headline={caption}). It is important that this field has not been used yet (in neither the JPGs, nor the NEFs!)
2. Apply this template to one set of files (e.g. the NEFs or the JPGs).
3. Synchronise the IPTC data. The caption field would now contain the "original" caption, the headline field, the "other" caption.
4. Set up an IPTC template that concatenates the two fields to yet a third (previously unused) field. E.g., Contact Email(s)={caption} {headline}.
You may need to choose your fields carefully (some have size restrictions)...
Hope this helps.