Go to Settings -> "Networks" -> "Add new network connection" -> "Tox protocol":
It will open "Network network" window, choose "Import configuration from file":
and point to the Tox ID file created by MirandaNG- I suppose it should be "some name.tox" and Isotoxin should be able to load it. I tested that with a Tox ID file created by qTox and Isotoxin was able to load it- please note that qTox and Toxygen use "name.tox" Tox ID files while Isotoxin uses its own "name.profile" Tox ID file which can be exported to "name.tox".
Besides Tox protocol Isotoxin also supports LAN protocol and Jabber protocol and "Isotoxin supports simultaneous connections to multiple networks".
I suppose that your Tox ID created by MirandaNG should not be incompatible with the current Tox core.
If you want to delete a Tox ID profile just go to Isotoxin folder and delete "name.profile" file corresponding to the profile you want to delete.
Warning!:
I hope this will help. If you encounter problems with Isotoxin please tell me.Important! New profile no longer compatible with old versions of Isotoxin's (profile will be automatically upgraded). If you want to use old version of Isotoxin, please, backup profile (btw, Isotoxin backups your profile every start, see backup path in system settings)
You may also contact the developer directly via the forum or via the contact page.
For me Isotoxin is the most advanced Tox client, for example see this:
and as far as I know the developer is currently working on new features, some of which will work only Isotoxin-to-Isotoxin because they will be client-specific, not supported by Tox core.