It looks like it is possible to convert TextMate bundles to Atom packages (see http://flight-manual.atom.io/hacking-atom/sections/converting-from-textmate/), but there currently seems to be no tool to convert from an Atom package back to a TextMate bundle.

It should be possible to do this by hand though, since it's basically just a different file format (property list instead of CSON).

Yes, for code completion, Textastic supports snippets including tab stops, placeholders, mirrors and transformations as described at http://manual.macromates.com/en/snippets

Textastic does support transformations in snippets, but I don't know if that this is what you are referring to.

See https://github.com/blach/Textastic-Customization#templates

Have a look at the TextMate manual entry on snippets for an overview of the snippet syntax. Textastic supports tab stops, placeholders, mirrors and transformations.

If this is not what you mean: Can you give me more details? What do these transformations do?

No, sorry. I don't use jailbreaks myself.