Adding speaker cab ohms advice needed

[wednesdaysworse]

I'm currently happily using an old Crate Vintage Club 30 amp with single 12", 8 ohm speaker (Weber). I would like to be able to simply add a 12" extension speaker cab when needed. There is no output jack for an extension speaker. What would be the safest way to accomplish this?

If I hook up a jack system and just plug in an external 8 ohm speaker cab, will it be in series with amp's speaker, giving me a "safe" 16 ohm mismatch? Will such a mismatch deteriorate the total sound a lot and defeat my purpose of "spreading my sound out" a bit on larger stages?

Any advice appreciated.