The total effective enhancement bonus cannot exceed +4.
The effective enhancement bonus, or modified enhancement bonus, on a magic weapon includes numeric enhancement bonuses to attacks and damage (such as +1, +2, etc) plus any special ability with an enhancement bonus equivalent.
Some magic weapons have special abilities [...] A single weapon cannot have a modified bonus (enhancement bonus plus special ability bonus equivalents, including those from character abilities and spells) higher than +10. A weapon with a special ability must also have at least a +1 enhancement bonus.
The purrodaemon's Weapon Steep ability includes an example:
The total enhancements cannot exceed a +4 effective enhancement—most purrodaemons opt to create +2 wounding weapons in this manner.
The wounding ability is equivalent to a +2 enhancement, so a +2 wounding weapon has a total effective +4 bonus. Weapon Steep has no prerequisite for the weapon itself, so these bonuses could be added to a mundane weapon, making it into a magical weapon.
As for your example, a +1 impact weapon would have a total effective bonus of +3. Weapon Steep could increase the +1 bonus to a +2. Alternatively, it could add one special ability with a +1 enhancement bonus equivalent, such as frost or shock.
The brilliant energy special ability is equivalent to a +4 enhancement bonus. A +1 impact brilliant energy weapon would have a total +7 effective enhancement bonus, which exceeds the limitations of what Weapon Steep can create. So it's not possible.
In fact, Weapon Steep cannot add brilliant energy (or any special ability with a +4 equivalent bonus) to any weapon. This is because these special abilities can only be added to a a weapon with an existing enhancement bonus of +1 or higher. At minimum, that would be a +1 brilliant energy weapon, which has a total +5 equivalent bonus. So it's also not possible.
Note that Weapon Steep probably cannot add special abilities that have no effective enhancement bonus. They don't count as "magical enhancements", as mentioned in Weapon Steep. Otherwise, the purrodaemon would theoretically add all of them, which contradicts the preferred +2 wounding ability cited in the example text. So it is likely that Weapon Steep can only add special abilities with equivalent enhancement bonuses.