Are there any software to digitize decorative stitches for the sewing mode in our embroidery cum sewing machines?

The answer to your question is Yes, you can. The Husqvarna software let you design stitches for your machine to be used in the sewing mode and as a fill stitch in digitising an embroidery design. You can upload it to your Ruby per USB stick. I have a couple I downloaded from the Husqvarna website (from their monthly newsletter, some Christmas designs) and some I digitised myself and I use those in the sewing mode.
I had a look and found an article on making decorative Motifs. It was for 4D or 5D but works the same in 6D.
And there is also a newer more concise one for 6D. In both cases choose Module Mystery in the Newsletter.