I've found a webpage that makes it sound pretty simple. On one of McCausland's Brain Imaging pages, they mention the process at the very bottom of the page:

You can also create your own anatomical template. The AALimage that is included is a nice example of how a custom template can be created. To create a template you should have:

- An 8-bit Analyze format image of the regions (e.g. a *.hdr file that describes the image dimensions, plus a *.img file that contains one byte of data for each voxel. There can be up to 255 different labelled portions (1..255, 0 is reserved for air and other non-cortical regions). Also, when first creating your header, it is important to adjust the header to denote that this image is a Template. To do this, open the 'Header Information' command from the 'Header' menu, set the 'regular [114]' value to 97 and then choose 'Save header' from the 'Header' menu. E.G.: AAL.hdr, AAL.img.
- A color lookup table [LUT, optional]. You can create a 768-byte RGB lookup table to describe the color of different regions (sequentially, 256 bytes of Red, followed by 256 bytes of green, and then 256 bytes of blue). This file must have the extension *.lut, must be placed in the same folder as the MRIcro executable, and should have the same name as the *.img file. E.G. AAL.lut.
- A text lookup table [optional]. Each line should start with the number index, and after a space have the name of the region, e.g. ...45 Cuneus_L, 46 Cuneus_R, 47 Lingual_L, 48 Lingual_R... This file should be in the same folder as the *.img file, with the same name except with the extension *.txt. E.G. AAL.txt.

I do not know how similar this process is to 3D Slicer (since I have little familiarity with that application), but that might be helpful. The INIA19 text file looks extremely similar to the text file McCausland's Brain Imaging webpage mentions. It might be as simple a process as updating the header information to something compatible with MRIcron.