Unless you have created the parts (using Manage Parts) as Linked Parts, you cannot edit or extract them, since extract parts just copies each existing linked part to its own file. Most folks simply use linked parts, as they reflect any changes made to the full score, but I realize with the phasing out of Finale, one may want to have separate files of each part until one has decided what notation program should succeed Finale.

My reason for extracting part files is I'm composing an orchestral suite which requires merging separate pieces into single parts for the instrumentalists. The depth of the orchestration varies from piece to piece (there are 14), and there is narration between pieces. I'm using Finale's MergeScores for this, and finding it dicey in some aspects: i. e. Perfectly formatted extracted parts lose some or sometimes all of their formatting in the merge when doing direct attachment. The "merge as separate movements" will retain formatting, but that's a waste of space if joining to a part only occupying, say, a third of a page. And apparently "separate movement's" systems can't flow backward to fill that space. The worst problem is sometimes a whole part is crammed into a single system in the merge, requiring manual down arrow separation of bars.

But I see no other solution for doing the mergers other than creating separate extracted files. Do you know of a better way?
Thanks.

No, not a better way, given what you need to do. Still, to extract all the parts, you will need to have linked parts for each instruments, if only to export them. That is something you can create in manage parts. No sense in trying to clean up the linked parts before extracting since you will have to format the parts again once you extract and combine them.

Best wishes for your project.

Thank you. I've already taken your manage parts suggestion and am working that way. One thing I do in the pre-extract part is  add measure numbers (each piece needs to start with "1", yet the whole of the final merge will actually have the sum total measures - it means working with the numbering regions.). Yet there is one bugaboo in the merge. Although I set a font, style and size of the font numerals, the numbering, size and style merge correctly, but for some reason the font defaults to Finale font numerals. That I have to fix in the final merge. And one other bugaboo: pieces that have been merged to will lose their final double bar line when a new piece is merged on to it. This has to be fixed manually (as well as disabling cautionary signature changes) with the measure tool. Extra labor is required if a piece happens to end with a multimeasure rest - that has to be broken to make the change and then  re-enabled. 

I've purchased Dorico, but I haven't even looked at it yet. I wonder how that platform would handle this.

 Again, thank you for your good wishes,

Have you thought of merging the scores first and then extracting the parts just to see if that avoids the double barline between pieces? You may find the amount of manual work is pretty much the same (even if the problems to solve or different) each way; but it's a thought.