Yes you can split that line, just make sure you can put everything back exactly as it was incase it dosn't work.(try not to cut anything

)
you would be better off bringing in a "home run line" from the box. The problem to the cable modem is not loosing to much of the signal that is coming in,it is making it too hard for it to talk back. ( signal getting back out too the pole, the reverse signal). you want the modem line to have the least loss of all the lines in the house.
So if you do run a line to your TV card, make sure it is split off after the split for the modem.The Ideal way to do it is to put a splitter ( or Directional Coupler ) after the ground block and a line runs from that to the modem. the other line from that splitter feeds the rest, ie splitters ,amps, etc.
If that can't be done,try to make sure the line the modem is on has the least amount of splits possible. and don't use cheap splitters.ie gemini and stuff fron the dollar store. there not made to handle the reverse signal. same for amps. splitters should be rated for 5 mhz to 1 ghz. Amps should say on them 5mhz to 40 mhz passive
(or something like that so the reverse signal can get back out).
hope this is of some help