You should of put a distribution amp/splitter on the system if there is more then One splitter. You dont want to put a splitter after a splitter the resulting signal loss or "db"s would be loss as to degrade the picture,, use RG-6 Belden. sometimes you can pull in your new cable by splicing in the old, its the old telephone splice.
Keep in mind this JUST for PULLING in the new wire. You strip both cables to the core the bend them 90 the twist in opp direction wrap with tape. Now provided he hasnt stapled the cables you should be able to pull the old out and the new in.
always use a good quality connectors and crip tool
NO RADIO shack stuff.
Lowes and Home Depot have them.
Oh if you do buy a Distribution AMP you need to Know the
Frequency Range 2-1300Mhz some need to be higher,another question do you have
Cable internet Or two way communicating cable boxes.
Ask you cable company If you dont Know.
the amp has to be a GOOD one if you need it at all
a good two way splitter with 4 out provided by the cable company
should solve you problems,
My system comes into the house then splits one to a two way then one for the cable modem