As I said, just tag um on every VLAN and away you go.
Or to answer your more direct question, a port an be an untagged member of only a single VLAN, however it can be an tagged member of an unlimited number of VLANs (well for the truly technical it can only be a tagged member of 4096 VLANs as that is how many possible VIDs there are. Hardware may limit you to even less.)