Dualboot on an external hard drive I have an iMac, its OS (EL Capitan) is located on an external hard drive. 
I want to install WIndows 10 as a second OS on this drive.

But, since it is an external drive, I can't devide it into several partitions, using BootCamp Assistant. 

Is there any way I can do it? Maybe I should use different software? Even solution, that includes erasing of the data is acceptable at the moment!

Thank you in advance!
You can actually divide external drives into several partitions. 

To do so you just need to add a partition table to the drive using Disk Utility. Unfortunately that will render existing data on the drive unreadable by conventional means.

So in essence you'll need to copy your data to a backup, add the partition table, format the macOS partition and copy back your data. Then you can format the Windows partition and install Windows.