According to Novell Cool Solutions article from August 31, 2005: NAT and

there is no way to connect an internal client through a Bordermanager server
to an external PPTP vpn host. A restriction in the way NBM handles NAT'ing
is listed as the cause. Has this changed at all with recent
versions/service packs or is there any workaround other than plugging the
client PC into the DMZ or public network?