Once you remove the DRM, then for:
1) .epub to .imp, see this
post
2) .pdb (ereader) to .imp, first convert the .pdb to .html as described
here, then use eBook Publisher to convert that .html to .imp as described
here. It's a two-step process and may need you to tweak it a bit to make it work.
If you had a unencrypted .pdb (PalmDoc) or .prc/.mobi (Mobipocket) ebooks, then my
Mobi2IMP would handle the conversion to .imp. Also,
lit2sb does an amazing job converting unencrypted .lit to .imp!
Enjoy!