Could you get pregnant by just penetrating once without condoms, and then using a condom to continue, as well as pull out before ejaculating?

Exposure to pre-ejaculatory fluid (semen) via unprotected penetration can cause pregnancy.

ANY AMOUNT OF SEMINAL FLUID THAT CAN COME IN CONTACT WITH VAGINAL FLUIDS CAN CAUSE PREGNANCY. SAFE SEX IS A WAY TO FEEL SAFE AND AVOID PROBLEMS. NO GLOVE, NO LOVE1

Any time a penis enters a vagina without appropriate contraception pregnancy can occur. It's that simple.