Not practical.
The bearing cups are fairly hard steel.
The U-joints that come with a center grease fitting also include passages through the "X" to channel grease out to the end bearings. A non-grease "X" is solid.
U-joints having one grease fitting on one cup have similar grease passages.
You may be able to slip a grease needle under or through the seal to inject a bit of grease into each bearing. That will be a temporary measure at best. You may damage the seal.
Even if you can properly grease the freed bearings...
If the U-joint seized the bearings are damaged. A bit of added grease may buy some time. The bearings don't see true rotating service, but even the limited movement on damaged bearings will eventually result in failure. Actually the back and forth limited motion service cycle may be tougher service for roller bearings than a true rotation. A catastrophic failure can ruin your day.
vic
