There are times when we may need to work in our model and use sheetmetal features for our regular parts. Either we are designing a model and a sheetmetal feature would help get the right design intent, or we are working on an imported features and would like to manipulate, adjust or fix them using a sheetmetal feature. I understand that a non-sheetmetal body shouldn't merge into a sheetmetal body. However, we should allow a sheetmetal body to merge into a non-sheetmetal body.
Below is an imported component that is a solid (non-sheetmetal body). If any of the flanges imported with an issue, one approach could be to recreate the flange with a flange tool.
Here is just a random model I threw together showing a flange being used in a non-sheetmetal part. I've had better examples in the past, where we have wanted to use a sheetmetal feature in a non-sheetmetal part, but I can't recall what they were.