There would probably be senior Allied officials receptive to such an offer, but in the popular mind Franco was too associated with the Axis for the public to accept it. The Allied leaders came under heavy domestic criticism for arranging a deal with Admiral Darlan during Operation Torch. It showed people did not want to collaborate with anyone tainted with working with the Axis. By 1944, it was obvious the Allies did not need help from Spain to win the war. Agreeing to include Franco in the Allies would give them little advantage while leading to immense criticism at home. FDR especially would be reluctant given the upcoming 1944 elections.
If there was no Darlan deal to give the Allies warning that any deals with fascists would be unpopular, then perhaps Franco could join.
It's also possible he could have joined in 1942 at the time of Operation Torch, although at that time it would probably be too much of a gamble for Franco since the war seemed to be very much in doubt.