They already are.................................. (but bear with me)
Before anything else - no there is no corrolation between players and officials wages. Players are employed by the club for whatever they are on, the officials are employed by the FA who under no stretch of the imagination have anywhere near the money available to them, farless to have it all in one invested intereset (club owner only as to think fo the club - FA has the whole game from grass roots to top to pay for).
Well if you think about it, if the PL ref's are on £38k basic plus match fees = £80 - 100k (HW declared £120+ in 2012-13) pa then they are onto a good thing and can afford to do it for a living.
Assistants not quite as much but probs a good £35 - 60 annual income. Either way, with expenses paid, these are good incomes and require no additional earnings to be comfortable.
Moving down..... more refs, more assistants, less of the cut to spend.
I don't know at what level people are paid retainers, or how much these retainers are, but in theory it should be enough that you can concentrate you time to the cause (ie of you got a £15k retainer and £150 a game, doing 30 games a season would bring in £20k.
Now, I'm no expert but if the average wage in the UK is around £26k (average wage = average job) you could argue that for 3 "days" work a week, £20k ain't bad, and ou could certaimly put most of your time and effort to the cause. Something else on the side (consultancy, coaching ya de ya) and we'er all good.
(Caveat = average being the total of all incomed divided by the number of earners. Thus for every person on £35k you would need someone on £17k to keep your average, and that less than 10% earn over £40k.)
But back to the OP - Professionalism is a phycological state, and is not a physical state. "Should National Group referees be professional like the Select Group?" I'd perhaps say that they are already. All because someone is at the top of their game or does something or a full-time job does not make them a professional. Numous cases from accross the busines worls have demonstrated this;
"Should National Group referees be paid, administered and treated like the Select Group, this be incorporated into the FA as full-time employees" - different question, to which my answer would be yes.