it depends on the version of client you are using, but sure.
start with the File/Edit/Tools ... toolbar, choose ACCOUNTS
then > account options
and on that menu, choose "Create account".
On some installs, you may need to configure telephony (why? I don't know.. just feed it an area code)... or you might need to update / reinstall WMS (Windows Messaging).
Just keep in mind that GroupWise is optimised on the client-server connection using port 1677 to a GroupWise mail server. It is still a very functional mail client, and can use any secure / non-secure POP, IMAP, and SMTP server you set up.
Before you do this, make sure you have your email in order. Keep folders sorted, double check your handling rules, or suspend them until you have the other server(-s) working.
Especially using POP, you might just get all your email dumped into one common ' INBOX'. You'll need to check the header of each email to see which of your accounts it originated from.
At my job, we do this sort of thing all the time. GroupWise is a primary client , and a lot of folks also choose to use Eudora, Thunderbird, SeaMonkey, Outlook, and Windows Mail / Mac Mail. Any of these clients can handle multiple servers.