Sorry,
I have not played HARP in a while so I forgot about the DB thing. Yes the DB to At IMO is going to be the hardest and I do not know of an easy way to convert right off the top of my head. If something comes up I will post it but I think you should just play it by ear a little. That is to say when you get RMC take a look at the armor table and they will have entries for varous animal armor types. I would estimate the HARPs monster AT an then take a way some from thier DB since it is going to be a lot higher. For reference I think there is a table in RMC that has varous professions and what thier OB, DB and skills might be. Use the numbers from that chart as a base. After a few games it should be easyier to guage what monsters might have.
Hits and OB I wouod keep tha same but you could adjust them by using the chart I talked about above. You are also right that spells is going to be the hardest but if you buy RMC Spell Law then it should be easier.
Also is there a reason you are not geting the combo-pack of RMC? As that woud give you the monster guide as well.
MDC