Yes, they can
There is nothing in the rules against it. It's only that feeling that all GMs get that creating magic items feels too easy. Well, there is any good reason for players to fail to create their own magic items and waste their Wealth by Level?
To create a magic item, you must spend at least half of its price as material components, meaning that you have to sell stuff to create stuff. Items, magic or not, are usually sold at 50% of their price. Meaning that to create another +1 weapon (2,000 gp), you must first sell enough loot to make 1,000 gp, which should be worth the same total of 2,000 gp. So, while it feels like they are getting what they want too easy, they are just making sure that they get something that benefits them instead of whatever loot the adventure gave them, not affecting their total wealth at all.
Anyway, casters may still cast all of their spells while creating a magic item, this is evidenced in the fact that the spells used on the item must be prepared (or known) during the process and cast every day during the process, not forbidding them to cast other spells, only that they have fewer spell slots available that day.
If spells are involved in the prerequisites for making the item, the creator must have prepared the spells to be cast (or must know the spells, in the case of a sorcerer or bard) but need not provide any material components or focuses the spells require. The act of working on the item triggers the prepared spells, making them unavailable for casting during each day of the item’s creation. (That is, those spell slots are expended from the caster’s currently prepared spells, just as if they had been cast.)
So, depending on their level and the number of spells available after spending the slots needed to craft the item, they must have enough slots left to be able to prepare those buff spells they want. If they have the slots, then they could buff themselves before making the check. Otherwise, asking for another character to buff them at the end of the process is still a valid option.
However, you do have a point that this doesn't sound right. What I ask my players, and this is a house rule of mine that I know some GMs also use a similar method, is to ask for those buff spells (or abilities) to be used every day during the process. So, if they do need those spell slots later that day during the crafting process, they won't be available. This effectively add another spell to the requirements, but they will get a small bonus to meet the DC they couldn't otherwise.
This should only be an issue if they cannot meet the crafting DC by taking 10 without such spells. Keep in mind that you may Take-10 on those checks, as officially clarified a few years ago.
I also know that some GMs will ask for such spells to last at least 8 hours, which is the daily time spent on creating magic items, so they can benefit from it. This sounds a little too harsh and there is only way I know this could work, by combining Crafter's Fortune with Master Craftsman. So, it is kind of a house rule forbidding buffs to create magic items in practice.
Ultimatelly, if the flavor behind the process doesn't sound right, just change the flavor of it. For example, make it so that this test is the final rune inscription into the item, as the finishing touch to complete a ritual. Similar to how the act of casting a spell is only the last part of a small ritual that you prepared earlier that day.
