I have an existing beam 4 x 8 is supporting 11 ft expand and I want to remove a pantry that is supporting this beam.

Now I want to put another beam under the existing beam that is an engineered
4 x 12 and want to extended to 17 ft long.

My question is how can I reinforce the floor that is supported in post and 4 x 6 girders?

You description of what you want to do is a bit unclear. "existing beam 4 x 8 is supporting 11 ft expand"? Please re-read your post. To support any weight down, which is what you have here, you need to find where the weight goes to the floor and install support back to the main beam or down to the basement or ground. You can go back to a wall and reinforce that but still need to go back to a foundation or footing, it just can't sit on the floor.

Your girders are under your joists which probably on contrete footers. You can not upsize the girder but can put more footers were you want to expand then add another girder Running off the end of the existing. So double up girder where you want support and off set your footings.

I could be wrong but that's what I got out that OP wants to do?