Can you splice a240v 30amp receptacle to move it further?

Yes, but you have to do it correctly. You should use 10 AWG wire. The splice should be in an approved electric box using the right size wire nuts. You secure and protect the extended wire and you should not "bury" the splice box in the wall, but cover it with an approved cover made for the purpose. If it is a surface mount box, cover it with a blank metal cover.