Here are the rules from Classic Traveller (from Book 1, p 8 and Book 3, p 22, 1977 edition) for when the players have their PCs hire a NPC:
Sometimes (often) players will encounter people not manipulated by an actual player. They may be thugs or assailants. They may be potential hirelings or employers. In any case, their skills and abilities should be determined using the character generation procedure, and noted for the effects they may have on play.
For example, a starship captain may be looking for a crew for his ship, in which case, the referee would generate characters until one occurs with the required skill (such as navigation, medical, etc.). Generally, the first appropriate character to be generated would present himself for employment, and if not accepted or considered suitable, an appropriate delay would occur before another presents himself. As an alternative, the referee might simply generate a character and assign him the required skill, plus perhaps 1 or 2 more.
When travellers require employees, for any purpose, they must find them in the course of their activities. This may require advertising, visiting union hiring halls, or active efforts in barrooms or clubs. Hiring is done by stating a requirement to the referee, who indicates persons presenting themselves for employment. The interview consists of generating the person's characteristics and experience.
These rules contemplate that NPCs may be generated using the PC-build rules, or may just be created by referee stipulation.
RuneQuest is a well-known FRPG which was one of the first to use the same mechanical framework for specifying all beings in the game system, so that they all feed into action resolution in the same way. But it does not require that all beings under the GM's control be generated using the PC build framework.
Personally I don't regard it as a very important desideratum of a RPG that a GM-controlled entity should be easily amenable to slotting into the player-side features of the game such as (eg) balanced character building, clear process for character advancement, etc. As I said the only version of D&D to attempt this is 3E, and it generated a lot of complexity (eg ECL rules) and weaknesses in design (eg undead with too few hit points because of their lack of a CON score).