1) I've been kissing the turba after prayer since I can remember, I'm not sure where I picked up this habit. Is this something that is okay to do? it consists of me kissing the turba then placing my forehead on it and repeating with my right cheek then left.
Also when in sujud I like to extend it with more zikr, one of the things I like to say is asraghfar'Allah 3 times, is this also acceptable?

2) Could you clarify where the kissing of the turba was started and what the intention should be while preforming it.

by

1) All what you do is accepted.

2) The kissing in directly comes from a Hadith attributed to Imam Sadiq in which he said: Allah has compensated Hussain for his martyrdom in that he made the Imams from his progeny and shifaa in his turbah!
Therefore from that Hadith we derive that his turbah is blessed and we kiss the turbah for the blessing found in it.