You need to be careful which type of grease you mix together. Dexter recommends that you use a lithium based complex grease. If you mix other types with lithium grease they may or will react with each other and cause the grease to fail. Many of the grease manufactures produce a lithium based grease you just need to make sure it's lithium based. If you can't find lithium based grease check with your local auto store.
I have attached a link from the site library at the top of this page. You will need to download the link and then open it. On page 21 you will find what Dexter recommends to use.
Google mixing greases you will learn what the grease manufactures have to say about doing this.
I pulled my brake drum off to repack the bearings for the first time this fall and found that 3 of the grease seals had failed and let grease into the breaks. I foolishly did add grease with the grease zerk to the bearings. It was only 2 or 3 pumps of grease. There was already way to much grease in the bearings this just made it worse. I will just clean and repack the bearings every 3 year or 12,000 miles by hand and never use the zerk again.
Hope this is helpful Tim