From the Swatches panel menu open the Ink Manager. From there you can alias any spot color to any other color.
You can merge spot colors using the Ink Alias feature of the Ink Manager. You'll find the Ink Manager in several locations, but the easier is probably from the Swatches panel menu. Select the ink you'd like to alias from the list of inks in the file, and assign it to the ink you want to output with.
After aliasing your inks, you can check your work in the Separations Preview panel (Window > Output > Separations Preview).
@P Spier and @Steve Werner
Thanks for your quick replies.
I actually knew about the Ink Alias feature (this is eventually the way I did it) but it is not that convenient
as I got around 30 spot swatches in my Swatches panel and a lot of aliases in the Ink Manager so it was at least confusing.
The Separations Preview does help but when actually designing is distracting to have all the imported graphics using different colors.
Again, thanks for your help.