If some mad hereteks would get the briliant idea of merging Tyranids DNA with his own or craft Tyranid chitin, claws, limbs, whatever into his body how would you say that this would affect his potential connection with the Hive Mind or the telephatic connection between Genestealers of the same brood?

I'm thinking that splicing with Tyranid DNA or merging brain-matter would suffice but in other regards it would not, yet I thought that I should look for some extra input before making a final call on the topic.

Already done in part with something from Seventh Retribution to make the ultimate assassin capable of absorbing biomatter from her victims. It made her into a powerful ruthless killing machine which went rogue and took a good number of veterans, assassins and champion marines just to kill her on the second attempt.

http://wh40k.lexicanum.com/wiki/Legienstrasse

The question depends how much you add. Unless you're merging the brain in though, your connection to the hive mind would be nil since only that has the connection. A limb is just a piece of flesh and bone without the brain to send it instructions so if you can find someone mad enough and skilled enough to somehow graft and get a scything talon fitted and have your body sustain it then fair play to you. It would be just like adding a servo arm only one made of the above. Unless you added the brain of a synapse beast then logically the hive would have no effect or control. After that though if you do, it depends what brain you added (warrior, gaunt, zoanthrope etc), how much of it and how strong willed the subject might be.

Note however that the bio-weapons (like deathspitters) are symbiote creatures and do have a limited intelligence (and hence hive mind connection) themselves...