Splicing in a LB, is it allowed by the NEC code?
Absolutely not.  No splicing in a conduit body. They are only to help fish wires.  

Now -- there is an utterly useless exception.  If the conduit body is stamped with a number of cubic inches, then you can apply the junction box fill rules instead. In any size of conduit that's reasonable for your wires, there aren't nearly enough cubic inches in the conduit bodies! 

You are better off just swapping the conduit body for a junction box. Play with several sizes of box to see which one will line up without having to cut conduit. Been there done that.