You should still be able to use it, but the yeast viability will be much lower. I recently had this happen; I bought some Roe back in February, and ended up fermenting a sour with my last packet of it in August. The smack pack barely inflated, so I bought a second 3763, and smacked that one, which was fine.
As jfolks says, you could make a starter, but I strongly advise against it. It's already hard enough to get a sour beer quickly out of Roeselare. It takes quite a bit of time to produce that traditional Rodenbach Flanders red sourness. If you make a starter, the saccharomyces in the culture will overpower the bugs, and you will end up with an even less sour beer. I suggest grabbing another 3763, or WLP655, which isn't exactly the same, but is still a culture of Sacc, Lacto, Pedio and Brett, like WY3763