Essentially, the only realistic way to accomplish this is going to be via bluetooth sending the contacts as vcards from the devices in question, as this is really the only ubiquitous generic format of transferring contact information around.
Using a spreadsheet via email would be an option, but you're still left with needing a way to get the contacts into a spreadsheetable format in the first place, and that's basically going to need several apps, one for each possible phone platform to transfer contacts from, and will require said app be installed on the phones to export the contacts.
Essentially, any single app to receive contacts will essentially just want to receive them in vcard format via bluetooth and store them into itself - Basically, a replacement of the n900's own contact system that's dedicated to "backed up" sets of contacts.
How easy that is I do not know, but frankly instead of having a dedicated application to receive the vcards and store them separately, your best bet is likely to be as festivalnut suggested, and just use mycontacts to bundle each set of "backed up" contacts into a group of their own.