Wallets created pre-0.15.0 are not HD wallets. I think the best thing to do if you want an HD wallet is to let Sexcoin create a brand new wallet (which will be HD by default), then send the coins from your old wallet to the new one. This CAN be done by juggling wallets and restarts of the client.

Because HD wallet keys are generated from a base key, importing old keys will not work.

EDIT: You can import old keys, but they will not be part of the HD schema.