I would not substitute in the same weight - as i am sure that plugs dont have the same efiiciency/utilisation that you get with pellets.
I would use 60% of the listed weight of the hop plugs.
Eg if it called for 100gms of hop plug i would use 60gms of pellets.
I am sure that others would be able to provide more accurate info.
You can use Promash to check it out - key in the hop plugs to give the ibu's and then key in the same weight of pellets to see the ibu's listed.
Then do a %calc of the difference and then that should give you the equivalent weight of pellets to use.
If they are for bittering I would think of a little less not sure how much I think promash adds something like 10% extra utilisation for pellets. If they are late additions I wouldn't stress over it as they wont be in the boil long enough to do all that isomerisation of the alpha acids....