Before you add new blood, have a question for you.
Do all your fowl perform the same way? Or, do you have some that perform properly?
If you get any that perform properly, then breed to those. Performance traits are selectable traits that can be improved through testing the progeny, selecting the the offspring that express the desirable traits, then breeding to those until the proper trait is exaggerated and expressed in future offspring.
I would only add new blood if the trait is nonexistant in your strain. And if you do add new blood, the new bloodline must not only have the desirable trait, but has the least number of contrasting traits that would change your strain too much. I would also create a seperate sub-line to add the new blood into, rather than changing your present strain. This way you don’t ruin the strain while your are doing experimentations.
Either way, I would like to talk with you one phone (coaching call) before you take such a leap.