How can I add another HDD as a slave.

It's not hard. Sometimes you need to change the positions of a jumper or jumpers that are on the back of the drive that you want to be the slave drive. There's a lot of information about adding a drive as a slave out there on the Internet.

Please click on the following search.

How to add a second hard drive as a slave. - Google Search

Once you have purchased your new drive, most of them have the instructions labeled on the sticker. Set the new drive as the slave and the other as the Master, windows will automatically recognize the new drive and you will then be able to use it :)