Of course you can, you can add the 100 amp breaker to your 200 amp as long as it isn't overloaded, to start adding the subpanel you must first calculate how many yards it will be from the main panel to the subpanel, there is a formula that calculates how much friction will be on the wire at the connecting distance, then you can look at a wire chart and choose the wire size that will best handle the resistance. Once this has been decided , depending on distance, you must figure where you will put this wire, are you putting it in the ground, if so you will have to protect it with conduit, and depending on how much and how heavy the the traffic that will cross the wires will be, if cars and trucks are to cross this area, it would be wise to get the proper conduit, or risk having to dig it all up because of broken wires. If the wire will be air bound you must make sure it will be out reach, to anything. Once all this has been established, you can now bring your sub panel in, you must then calculate the amount of constant amprage that will run through this box, it is best if the constant amprage falls short of the total amprage by at least 20 amps. Wires tend to heat up when the amprage is raised. All that is left to do now, is connect it all together to complete the curcuit.