How to append condition to existing expression

I have Linq Expression and I want to append or || condition to expression if true.
Expression myexpression;
myexpression = x => ||;
If(×.vendorId != null)
{ myexpression = x => || ||; }

Above I reapeated same expression and append extra || or condition. I have so many conditions to check.
Is there any other simpler way to do it.So that I can get output like below.
myexpression =myexpression + ||

Ashish Mayekar