== I have 20+ blogs written that have been shared all over the internet via default permalinks. ==
WP/BP versions? Are all those 20 blogs in a multisite installation or in single WP’s?
== Is there ANY way to override this or to set the custom structure so that it is exactly the same as the default? Any help would be great!! ==
It all depends on the configuration of your blogs – MS or single, but there are ways to go about this.
On multisite installation, just create another site e.g. http://mainsite.com/newsite where you can define BuddyPress and set permalinks to other than default.
On single WP, e,g, http://example.com, I would create a physical subdomain http://buddypress.example.com/ install WP at root using the same database as the example.com site but with a different prefix and share userbase — backup DB first before attempting this.
http://codex.wordpress.org/Installing_Multiple_Blogs#Multiple_Databases.2C_Same_Users
No, BuddyPress absolutely requires custom permalinks. What about putting “%post_id%” into the custom field? I’m not sure what the default behaviour is when permalinks are not set, so you may need to play around — take a read of http://codex.wordpress.org/Using_Permalinks#Choosing_your_permalink_structure