The X68000 core currently only supports .d88 tape format and .hdf hard drive formats. Converting a tape to a hard drive format isn't an easy thing to do, and involves copying the operating system files, removing unnecessary files, installing the game, and writing a custom boot script that is different for every game and takes a lot of troubleshooting. Thus, you need to understand some inner workings of the computer in order to do that. However, converting a tape format to another tape format is feasible, so I wrote this short tutorial for those that don't know how to use a .dim or .xdf file with the MiSTer X68k core.
First off, tape formats. Tapes are tapes. The format may be different, but the data is there regardless. As long as the general size is the same, it should be able to be converted. Files may be slightly smaller or bigger depending on padding or extra metadata, but generally a tape for our use case is around 1.2 MB. .d88 files end up being 1.22MB after conversion.
While the MiSTer core only supports .d88 tape formats currently, you can convert the large plethora of different formats to .d88. New format support hopefully will be available in the future, but for now we have to make do. (Honestly I really hope for .HDS large size hard disk support more than tape image support, as you can't even connect large hard disks with the core currently.)
Fortunately, there's an easy way to convert most (if not all) known formats to D88. Go here and download vfic.
Download Button Screenshot
Extract the zip and run the vfic executable. Use the "Convert" tab, make sure the bottom-most "FORMAT" dropdown is set to D88 first, then click the folder icon to select and open any tape image. It should say "succeed" in the log, and you'll find the .d88 file placed next to wherever the opened image was located.
Hope this helps.