The Wisp has Incorporeal Movement
The will-o’-wisp can move through other creatures and Objects as if they were Difficult Terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.
So it doesn't seem like the wisp can be kept in the bag against its will - it can just pass through the walls of the bag. What happens then...is best left to a separate question. ('What happens when a creature with Incorporeal Movement leaves the confines of an Extradimensional Space?')
Wisps - you can't live with them (Consume Life), and you can't put them in a sack (Incorporeal Movement)!
Not grappling
While there is no RAW way to put the wisp (or another Tiny creature) in the bag of holding (or another bag), I would not suggest using the grappling mechanic to achieve this. As written, the Grappled condition assumes an active attacker holding the grappled creature in place by using their hand (the condition ends when the grappler is incapacitated or the creature is moved beyond the grappler's reach - neither of which seem to apply to capturing something in a bag which could be closed and left unattended).
An Improvised weapon
An Improvised Weapon:
includes any object you can wield in one or two hands...Often, an improvised weapon is similar to an actual weapon and can be treated as such...
Using the bag is closest to using a net, since a net is used to capture not damage, and a target hit by a net is restrained
A restrained creature’s speed becomes 0, and it can’t benefit from any bonus to its speed.
Attack Rolls against the creature have advantage, and the creature’s Attack Rolls have disadvantage.
The creature has disadvantage on Dexterity Saving Throws.
These conditions reasonably represent the condition of being confined inside the bag.
It is the GM's call how similar the bag would be to a net, and which of a net's properties would be used when attacking with a bag as an improvised weapon. Personally, I would:
Retain the restrained condition on a hit. However, note that a bag cannot be seen through, so the advantage to hit the restrained creature would likely be offset by disadvantage for attacking an unseen opponent.
Remove the proficiency bonus to attack. Unlike a net (which is a martial weapon), creatures will not have a proficiency in attacking with the bag.
Retain the limit on one Attack per turn. Along with not being proficient, creatures should not be able to use a bag to make Extra Attacks.
Possibly make the bag a melee, finesse weapon. Nets are ranged weapon, but a bag is not made to be thrown, so should be counted as a melee weapon. Depending on technique, either strength or dexterity might improve your chances of getting something into the bag. However, note that one always has disadvantage to attack with a net (since you are either attacking at long range or at 5 feet or less with a ranged weapon). Bags should not be better than nets, so I would either rule that attacks with a bag are at disadvantage or that you cannot apply an ability score bonus to the to-hit roll.
A net can be used on size Large and smaller creatures. I would restrict the bag to size tiny creatures.
