Thanks for all the advice, but I was not explicit enough.
I know how to import into one table (I use EMS Studio as my MySQL Manager), but I don't know how this application structures its database. A lot of applications (e.g. phpBB) split the data relationally and introduce many additional specific fields. If PowerMessenger does anything like that, then a simple import will not work.
I don't want to buy a licence and then find it's no use to me. I'll stick to GroupMail if that's the case.
I'm afraid the pages on the WebAssist site don't say very much about how it works. I like to be able to read the Manual.