Use another production, say ret_stmt, which can be either empty or contain a single return statement so in your .cup file you will have this productions:
ret_stmt ::= // empty
{: /*your action for empty return statement*/ :}
// Single return statement
| "ret":r TYPE:t
{: /*your action for single return statement*/ :}
PROC_DECL ::= "proc":p ret_stmt:r NAME:n
"(" param_list:pl ")"
"{" { DECL } { STMT } "}"
{: /*your action for procedure declaration statement*/ :}
You can use a similar approach with parameters declaration, adding the production param_list.