I found a way to do this so this is a short set of instructions. This solution is far from perfect and neither are my instructions but I hope it'll be of use to someone who'd like to do the same, i.e. use their old phpBB userlist with Drupal and still keep phpBB as message board.
You'll need the following:
Arne Kepp's module, found here: http://drupal.org/node/32818
An existing phpBB forum
A Drupal installation
BEFORE YOU START: Make backups of your phpBB and your Drupal databases!
Start by reading the README that comes with the module above, do so very carefully.
I'd also like to advise people against trying this unless they are able programmers. It gets technical!
The module sorts everything apart from importing old users from phpBB to Drupal. This was I had to figure it out for myself. This presents a problem as both Drupal and phpBB makes the first user admin however they have different IDs (IDs or UIDs are used to indentify records in a db table), as illustrated below:
Trying to import into 4.6.3, which is remotely host.
I could not get the XML import to work, so I tried CVS. Not only did that not work either, but the books I had stored are now gone.
These are the errors I'm getting:
CVS import
Node Type -> Book
[check] Insert non existing records
Processed 0 of 39 line(s)
#1: Cannot force new node id nodeid 0
#2: Cannot force new node id nodeid 0
#3: Cannot force new node id nodeid 0
#4: Cannot force new node id nodeid 0
#5: Cannot force new node id nodeid 0
#6: Cannot force new node id nodeid 0
#7: Cannot force new node id nodeid 0
#8: Cannot force new node id nodeid 0
#9: Cannot force new node id nodeid 0
#10: Cannot force new node id nodeid 0
#11: skip nodeid 12
#12: Cannot force new node id nodeid 0
#13: Cannot force new node id nodeid 0
#14: Cannot force new node id nodeid 0
#15: Cannot force new node id nodeid 0
#16: Cannot force new node id nodeid 0
#17: Cannot force new node id nodeid 0
#18: Cannot force new node id nodeid 0
#19: Cannot force new node id nodeid 0
#20: Cannot force new node id nodeid 0
#21: Cannot force new node id nodeid 0
#22: Cannot force new node id nodeid 0
#23: Cannot force new node id nodeid 0
#24: Cannot force new node id nodeid 0
#25: Cannot force new node id nodeid 0
#26: Cannot force new node id nodeid 0
#27: Cannot force new node id nodeid 0
#28: Cannot force new node id nodeid 0