Are you using sde? If so what about using a database tool (we use TOAD) and just modify the table outside of Arc. It is safe as long as your tables are not versioned or if they have no add/delete tables. Or you could use arcpy with ArcSDESQLExecute and do the same thing.
You can edit your versioned data via SQL in SSMS. Please read all the following documentation that we have on it and you should be able to edit the data. I am not sure about the joined table.
Hope this helps!
Is it a one to one relationship? If so you could use a temporary join to do what you want. SInce I don't know how the table you want want to attribute is defined, you may need to add a field (or more) to hold the data you need.