You can use Copy-Drop instead of Move-Drop to add files to the list instead of replacing the list.
You can use the modifier keys (Shift, Ctrl, etc) to change the drop modifier. Depending on your operating system, it'll look and work slightly different.
https://ux.stackexchange.com/a/83769
Drag and Drop queueing feature
Re: Drag and Drop queueing feature
Please read the FAQ and How to Request Help.