Is it do-able? Yes
Do the airlines take helo hours into account? Depends, if they are multi crew then definately
Can you 'jump' into the Airlines directly? Yes, I did with a major carrier
You WILL need a CPL(A) and in the UK under JAR there is a possibility to convert an ATPL(H) to ATPL(A). Ask Alex Whittingham at Bristol Ground School, he can give you the details.
You will still need 100 PIC of fixed wing aircraft and a multi IR before issue of your CPL(A). Once you have accrued the minimum hours required for an ATPL upgrade this can be done with 50% of the hours required permitted to be on helos.
Have fun