I'm also using the SET Editor B085 for the set file. You can open the set to be edited and then it shows an "Import Tiles/Groups"-option. Here you can open a different set file, choose tiles or groups and then just select wanted parts to be merged into your set.
BUT watch out: The SET Editor B085 deletes parts of the set file - those, which came with 1.69 (maybe earlier), eg. SelectorHeight=...!
So, backup your setfile, you're working with, before using SET Editor B085! After the edit, I check, what has been taken out (check until the tile entries start). I usually just copy back in the top part.
For the itp file, I use the modified version of the GFFEditor. If I remember right, Zwerkules uses TlkEdit13b. It can read (and write) itp-files (just checked),
Another word of warning with using SetEditorB085, is that it has absolutely NO IDEA that and "a" and and "A" are actually different and will convert ALL pathnodes and visibility nodes to lower case, thus completely destroying any effort to use tiles that are using ANY of the capital letter path, visibility nodes. And that is not an easy search/replace that any text editor can use, you would not want to wantonly change all lower case to uppercase either as they do mean different things. I believe it can also corrupt some other bits on individual tiles beyond just path and visibility nodes, but I can't remember exactly what those other changes were.
SetEditorB085 was great in it's day, and still has some uses for EXPORTING tiles / groups etc, but it destroys any true .SET file when you export the entire thing. I use it for finding groups, and for displaying certain aspects of tiles etc while working with a .SET, but beyond that, I have found that I must manually edit a real .SET using a text editor if I don't want things damaged. That of course means learning a bit more maybe, but it is worth it in the long run.
The major damage it does is not something that is even visible via toolset or game, UNTIL you attempt to have your NPC's run around an area and they get stuck all the time because the engine can not find the proper pathway through a given tile that is using the lower case letter for a tile when it truly needed the upper case version. ALL of those changes were implemented with 1.69 and beyond, so any tileset created since 1.69 was released may not and probably will not work correctly if re-exported by SetEditorB085. Just be warned and careful, and do your actual editing of the .set in some other program and you should be ok.