Please think very carefully before attempting to melt glycerine suppositories in a microwave oven.
(1) The small suppositories will not provide enough "load" for the oven to work properly and the oven might stop heating to prevent itself from being damaged by an overload caused by being underloaded. IE, not enough water in the load to absorb the microwave energy and allow cooking, thus underloading the oven. This might sound ridiculous at first - but in microwave theory, any underload is an overload.
(2) The way the heating effect is distributed inside a microwave oven is very uneven, and there will be waves of intense heating effect moving around inside it. When these strike the suppositories they can easily boil the glycerine in an instant, which is not good.
By far the safest way to melt glycerine suppositories is to heat some water in a saucepan on your stove. Then put the suppositories in a small stainless steel cream jug and sit the jug in the hot water until the suppositories have liquefied. Stirring them with a teaspoon to break up lumps speeds up the melting.
To make a super-suppository you can then add some extra liquid glycerine to the melted mix, and then pour it into a mold and place in the fridge to set.
But there is a limitation on just how much liquid glycerine you can add to the melted suppository base before it gets too diluted to harden properly. If it doesn't harden properly, just remelt it and add more suppositories. And then try again.
In the 50's, suppositories were hollow in the middle, like penne pasta. This was done to make them melt faster in the rectum, and to eliminate wasting ingredients by not having a centre core in the suppository. Because the centre core of the suppository gets expelled before it melts all the way through anyway. So why waste the materials to provide it?
Wow, hi-efficiency suppository technology from the 50's. Pity they don't make them like that anymore. But this technology exists and it's fun experimenting with it.