In BRP there is less need to create endless reams of new monsters. Cos the monsters are usually people too!
Any intelligent or even semi intelligent creature may be a cult member, and layering a cult spells and items and stuff all over one broo or orc makes it quite different from the next one.
Another thing to remember is that any BRP creature grows with experience just as a PC. So again your orc, who may be a cultist in a death cult, maybe 25-30 years old, and will have experience and skills to reflect that, not to mention spells and magic items. He may also be utterly different from another orc from a tribe that worships the moon, tends to have madness spells and a propensity for using spears and arrows, and wears lunar silver armour..
As always, keep the opposition much weaker than the PCs, so a part of 4 PCs may go up against 4 orcs, 3 of which should be crap, maybe 25% in weapon skills, and minimal armour, maybe 1 low powered spell between them. The 1 who is good, can be at the same sort of experience and skill mix as the PCs, but with interesting spells or one nice item. The PCs wil kill the cannon fodder and you decide as a GM if the tougher guy stands and fights or runs or offers ransom for his life if he surrenders.
Now, dumb monsters.. look at previous examples in the rulebooks you have, or some of the older BRP books, or maybe Monster Coliseum for RQII/Legend.. and these critters that are non sentient will be more instinctive, their stats and skills are always the same, and you can model their funky 'effects' from spells or powers (but they're probably not magical).
When converting a D&D monster, be very very careful. A critter built from BRP can be lethal, but also a direct mapping of D&D skills and powers to BRP can make for a TPK! So.. best to get a feel for the system and your players before making a new critter, just tweak the BRP ones for now.