The phrase "to supplement with formula" means to provide additional nourishment or sustenance, typically in the form of a liquid dietary supplement. Full definition
I needed to start supplementing with formula when my daughter was 4 months old because I did not make enough milk no matter how hard I tried - I cried. (rookiemoms.com)
We tried supplementing with formula for the first time this morning and my 10 month old wouldn't have it. (exclusivepumping.com)
Plus: Get tips and advice for supplementing with formula if you have a breastfeeding baby. (babycenter.com)