I’m pretty sure you can’t do this.
Plugged vias require an extra process step – somebody has to go in there and squeegie the special epoxy into place. The epoxy is also very specialized stuff… it’s not too expensive, but you have to order large quantities, it has to be kept refrigerated, and even then it only lasts about three months. I actually had to get some and it was a really difficult process.
SOOO…
If Seeed doesn’t prominently advertise this, they probably can’t do it.
I really wish they did offer it. I would be happy to pay twice as much to get via-plugs on my boards. For BGA components it lets you do via-in-pad, which is like getting an extra 1.5 layers for free. So your 4-layer boards are basically 5.5-layer boards, but they cost a LOT less than true 6-layer boards.
Please consider offering this, Seeed!
PS, if you were thinking of using soldermask to try to plug the vias, forget it. It does not work AT ALL. It used to half-way work back when very old soldermasking technologies were still in use, but it doesn’t work at all with modern soldermasks.