mixing solids with formula but..

Carms💜 • 31💜 11 years married💜Boy Mom💜8,7 & 1 year old💜
So my baby is 4 months and want to start solids. But I have no pumped milk anymore in the freezer & barely can pump anything anymore.. Is anyone mixing a little formula for the solids with a baby that exclusively been breast fed? I just don't want his tummy to get upset😞