It is possible to have AppExchange packages made available to Community Cloud users, however the cost of doing so will depend on the package licencing model. The trickiest option is likely to be with high volume licences like Customer Community Login as these are not based on number of users, but numbers of logins per month.
I would advise starting with the Salesforce Communities Implementation guide and testing in a dev org first as it is possible to create a community and some users for it in the dev org and then getting an understanding of the differences to working with regular user licences.
Communities Licence types documentation states that the objects in Managed Packages (which will include all AppExchange Apps) do not count against the Community licences Custom Object limit. If the App requires access to standard objects then you need to see which objects are required as Community Users access to these is controlled by licence types, e.g an App which requires access to Opportunties could only be used by Partner Community users not Customer Community users.
Installing an AppExchange package would be as normal except when you reach the stage of installing it you either install for all users or include the necessary community profiles in the selected ones.