A photo would help, as one man's full is another's "plenty of space"
Yes, it can be. If your frames have eyelet holes in the side bars, consider wiring the empty frames with beekeeping foundation wire or nylon fishing line. The bees will build that into the comb and give it more support.
Were all of the following true when you added the Flow super? Only add the new box (filled with frames) when all of the following are true:
1. Every frame has fully drawn comb, or at least 90% drawn comb - no huge gaps, and
2. Every frame is 80% full of honey, pollen or brood, and
3. Every frame is completely covered with bees.
If all of those were true, did you try waxing the plastic Flow frames? The bees take to the super much more readily if you do.
It may slow them, but should not exclude them. Rusty Burlew posted a recent blog article about using a top entrance during nectar flows to increase honey storage. You might want to consider that:
Please use the upper right magnifying glass tool to search this forum too. Many of your questions have been answered several times already.