You will not be able to merge multiple groups into one big entity. However, please read below to know more about this and check if this workaround is helpful.
As you grow and accustom yourself to using our product for your portal, there will be some changes made at your end to alter group composition and move agents according to their field of expertise. When you are looking at combining groups, what we could suggest is having one parent group called "Support" or "primary service" with all the agents in the smaller groups that you want to merge together. This is because even though you cannot merge groups, you have the functionality to associate the agents to multiple groups.
To add a new group, please navigate to Admin -> Team -> Groups -> kindly click on "New Group" to facilitate this.