You asked about plugging something into your existing keyboard. The biggest problem would be managing to get the bass of your keyboard to not make it's normal sounds. I'm not sure if that's possible.
If it is possible, there are a few options. First, Roland makes something called the BK-7m Backing Module. I've never worked with one, but it's something to look at.
https://www.roland.com/us/products/bk-7m/
There's also an Italian company which has some very high end arranger modules. The company is called Keytron, and they have four different levels of arrangers available. One problem with Ketron is that they aren't well distributed in the United States (where I live), although I've heard some very good things about them.
http://www.ketronmusic.co.uk/products/Ketron/arranger_modules/index.html
If you demand an 88 note weighted keyboard, you're not going to find fully functional arrangers. The DGX series that SeaGtGruff mentioned aren't bad, but one thing that arrangers do that most keyboards don't, is have a huge number of big buttons for modifying things on the fly. The DGX buttons are small, and harder to hit on the fly.
For example, I lent my Tyros to my piano teacher for Easter. (She couldn't find a trumpet player.) It was very easy to show her how to transpose the keyboard since there were dedicated transpose buttons. The sound selection buttons were available for each type of sound, so she pushed the Brass button, then scrolled through the screen to find the perfect trumpet. If she wanted to use the accompaniment, there are large, easy to use buttons to switch the variation of the style being played. And the default is for one sound on the left and three on the right. You can easily pick which sound plays by pressing a button for each of the three sounds. If the sound is on, the button lights up.
All of these are to make it easier for the live musician. And (as far as I know) none of them come with 88 keys.
Both Korg and Yamaha make a 7X key version, with Korg's being substantially cheaper and more portable. Like I said, Korg's giving Yamaha a run for their money.
Do you have a specific budget in mind?