Incorporating old mail?

[kjell.myksvoll@telenor.com (Kjell M. Myksvoll)]

Is this possible with nnimap?

I tried the description given in the Gnus documentation - created a
doc-folder of the mbox file with the "G f" command, marked it with 
"M P b" and tried to respool the messages with "B r". Unfortunatly
nnimap did not showed up as one of the possible backends one could
respool to.

Is there any other way to incorporate old mail in mbox format into
nnimap? I'm still using pgnus-0.84 (all patches applied) with
nnimap-0.114. Perhaps I should upgrade?

best regards,
kmm