Femap doesn't generate any midsurfaces using any of the midsurface commands for this IGES model. May be it is an issue with the native geometry itself.
However, I can generate most of the midsurfaces in MSC Patran. I am attaching the IGES model here.
Also, how can I "join" any of the new midsurfaces/surfaces I create to the ones which were generated using automidsurface command?
IGES is a valid format for surfaces, yes, but to import genuine solids STEP is better. Please note after importing the IGES file what you have in FEMAP are surface "sheet" bodies only (use MODIFY > COLOR > SOLID > SELECT ALL > RANDOM), not any 3-D solid exist, then MIDSURFACE EXTRACTION using automatic command is imposable.
What I should do is to recreate the 3-D SOLID body in FEMAP based in the imported sheet solids: learn to use command GEOMETRY >SOLID > STITCH, is very powerful!!
For instance, I will move all surfaces of the same part to a new LAYER (use comand MODIFY > LAYER > SOLID), the result could be the following picture:
Next I will use command GEOMETRY > SOLID > STITCH and select the all the surfaces in the layer (make sure to use a tolerance 1e-4) and the result will be a perfect 3-D SOLID!!
Solid Stitch 29 Surface(s) Selected... Surfaces successfully stitched. New solid conforms to Parasolid modeling tolerances. Created 1 connected region(s). Created Solid 29. Solid 29 passes Body Checking
Go part by part, at the end you can joint all solid parts in a solid (GEOMETRY > SOLID > ADD), or perform MIDSURFACE EXTRACTION part by part.
NOTE: I strongly suggest to create a new LAYER named MIDSURFACE and then perform MIDSURFACE > AUTOMATIC command part by part, the resulting midsurface geometry will go to the ACTIVE layer, ie, the created MIDSURFACE layer, this way you have items separated to allow an easy management: solids in the default layer#1 and midsurfaces in MIDSURFACE layer, OK?.