B6000 - convert oil hubs to grease?

Your plan is an OK temporary solution. It's been done in many situations with similar type gears buttt..... again this is at best a temporary solution.

But that being said...... if things have been dry awhile due to poor maintenance your seals may still be good...... they just need to re-swell. Put oil back in them several times and use the tractor. With time as the oil soaks into the seals they may re-swell and seal back up. If this does not happen then put new seals on order and fill up the gear area with Grease. Put on your calander a time to get the new seals swapped in and anything else that could be done while in there (bearings?)

I have seen the grease thing done before, and it's undoubtedly superior to no lube, but I'm just finishing a complete rebuild/repair of my B7100 front axle. I would guess yours is similar, and the thing that would concern me is the ball bearings. The gears would probably be all right, but I would worry that the grease would not reach or stay in the bearings, especially if they have cages as tight as the ones in mine. I think the only way you could get the same "flood lube" effect that you get from oil would be to 100% completely fill the cases with grease. (no air spaces) That way as grease was driven out of the bearings, other grease would have to flow in. Might make an interesting research study.
When we wanted to fill a worn differential with grease, my industrial arts instructor (back in olden times) used to say "It was engineered to run in oil, what makes you think you can improve on that?" Let us know what you decide. I'd be interested in the results if you try it.

If the seals are just dried out, I have had some improvement in such things by adding 20-25% ATF in the oil- ATF has pretty good seal conditioning properties. I wouldn't run it permanently, (too thin) but as a temp thing it might be worth a try.

Rubber seals run dry are burned off. There is nothing that will make them swell to seal again. Seals are cheap, any good auto parts jobber can measure and or change up numbers to get one that fits.

Grease- ok but its open to differential end (correct me if im wrong)and oil will find its way through.
Shaft where seal runs can be restored with a speedi sleeve.
If i was to use grease,00 grease can be found at a snapper lawnmower dealer. Its grease thats kind of runny.

Hope you find a simple solution.

It looks like your B6000 has the same "ball knuckle" style axle as my B7100- if so, the knuckle seals are probably the main leak (#72,73,75) and won't be found at any auto parts stores- dealer only.
For mine they ran a little over $53, from Coleman. Assuming yours is like mine, if necessary, one could fabricate the gasket, (#75) and the felt wiper (#72), but I see no practical way to fabricate the rubber seal. (#73)

As Tooljunkei suggested use 00 (some call it corn head) grease. It will fill into the small spaces better than normal grease but will not run out like oil.

I own a B6000 4wd and have rebuilt the front axle on it.

I do not think your proposed solution using grease will work well in the longer term.

Why not clean up the axle well with petrol or whatever. Then refill and identify exactly where the oil leak is.

That will enable you to make a better judgement on the cost of a proper fix.