It is possible. I purchased a 30-day Temporary Membership at the end of February at Centerpoint in Atlanta and specifically remember being told that I had 30 days PAST the date of expiration in which to apply my $90 toward the purchase of a One Year Membership. I was a Base Member at the time but knew I'd hit Silver sometime in early April...so, armed with what the Lounge Staffer in ATL had told me, I planned to hold off buying a Full Year until I attained Silver and could save $50 off the price that a Base Member would pay (plus my $90 from the 30-day).
Fast forward to early April (I was Silver by this point). Had a SLC-LAX-SLC trip and before my outbound to LA, I went into the SLC SkyClub to buy a One Year membership, thinking I'd be able to apply my 30-day and get $90 off. Wrong. I was told that I had to buy it within the 30-day period and couldn't apply anything now--I was too late. So I got on the plane, flew to LAX and tried the same thing at the LAX club. Again, same response, no dice. As soon as I got to my hotel, I did some digging and found and printed off the following:
http://www.nwa.com/services/bustrav/...empmember.html
Scrolling down to point 12, the T+C's state that, "The cost of a 30-Day membership may be credited within 30 days of expiration toward the cost of an annual or three year membership."
Upon return to SLC and seeing what I had printed off, the SkyClub rep was happy to give me a $90 credit (however, when I asked if I could use the 20 percent off April promo, I was told I could only use one discount at a time).
I hope this helps. Good luck!