add constraints?

Hi,

How may I add multicolumns primary key to the existent table and then
add foreign key constraint references to it for another existent table?
In this case I can only add primary key by creating unique index, right?
May I count if I create multicolumn (unique??) index the next command

alter table <tblname> add contraint <constrname> foreign key
(..) references <that_indexed_table_name>

will treat that index as primary key definition?

Thanks,
Dmitry Sorokin
ds(at)ics(dot)elcom(dot)ru