convert 12 word seed into 24 word seed
This is not possible.

The mnemonic words are an encoding of the entropy used in the seed, essentially. A 12 word seed has less entropy than a 24 word one, so you cannot get 24 words out of it. Similarly, you cannot get a 24 word seed to turn into a 12 word one.

Your best bet is to generate a new 24 word seed and move funds over.