Based upon the limited information posted by the OP it is theoretically possible to weld a tang on and make some sort of fixed blade, but it may be fraught with problems too. Setting forth in optimistic ignorance may be open minded and adventurous to some people and fool hardy to others. I like to hear what the pitfalls could be before I start something so that I can make an better informed choice about giving it a go; if it is possible, but likely to be risky, complicated, or time consuming to achieve what I want, is that going to be worth my time and effort?
Welding hardened alloy and stainless steels can be done, but the person doing the welding needs to know what they are doing with those particular parameters. If they don't, they may over heat the blade and remove its hardness, or produce a weak and/or lumpy joint. The weak point on most knives is right around where the blade enters the handle, right around where the weld and neighbouring heat affected zone will be. In the early days of me making knives I had two O-1 blades treated by two very experienced blade makers. The first over tempered the blade and then told me I hadn't provided him enough material data to do the job right. The second over heated the blade and quenched so fast that it broke with awful grain structure, at which point he announce he had never tried HTing O-1 before. Really skilled people can still muck stuff up if it is outside their usual experience. Forewarned is forearmed.
If I had a folder with a "lovely" blade, but "flimsy" handle I would be thinking about how to upgrade the handle before I thought about making a fixed blade out of it. I would also question how good a blade really is in a folder with a flimsy handle. The folders that I have had have either had good blades and good handles, or had poor blades and adequate handles, in the latter case they were never of sufficient monetary value to make it worth spending time turning them into something else. I have made both replacement blades and replacement handles for good folders.
A little more information from the OP would help and improve the value of advice.
What knife, what steel, what has it been used for till now (what is the yard stick for blade and handle quality) and what use is imagined for it post conversion?
Best of luck whatever...