Is it possible to become pregnant by pre-ejaculate?

Yes. Sperm is sperm, no matter when it comes out.

Precum is What is known as "cowper's fluid" and this is a mucous material this made without sperm, However it is not unheard of that cowpers fluid has become contaminated with sperm from a previous ejaculation and has lead to a slight risk of causing pregnancy. Sperm is produced in the testicles and mixed with materials produced elsewhere to make up the semen that is ejaculated with orgasm.