In Spain, 'Imma' (Catalan) or 'Inma' (Spanish) is only associated with the name 'Immaculada' or 'Inmaculada' from 'Immaculada Concepció'/'Inmaculada Concepción', 'Immaculate Conception (of Mary)' in English. I think every woman named Immaculada is called Imma. So we have both Emma and Imma, being completely different names.
Being of German heritage and very fond of their culture and different names, I'd have to say this is not very pretty, or just not very complete. I think Emma looks a lot prettier.
Please, if you want to name your child this, spell it with an E. If you think about it, she'll be teased and kids would say "Look, there goes Imma-ture!" Unless it's a middle name.
The suggestion of "Tbird" was good but unfortunately he/she did not say where to put the "e" as both "Emma" and "Imme" (meaning "bee") is possible. Anyway, I don't know of anybody getting called "Imma" here in Germany...
Catalan pronunciation is /'im.mə/ [noted -ed]