Does anybody know, aside from doing tension swatches, how to convert a standard sweater pattern to a tuck stitch pattern? I’ve just knitted the back part of a jumper on the machine in tuck stitch but it has come up wayyyyyy too short… any advice please? 
You will have to do a lot more rows in the tuck stitch. It makes your finished fabric shorter for the same number of rows. It also means that it is a bit wider. Launder your finished swatch the same way you plan to do with the sweater. Then find your row gauge so you will know how many rows to knit for the right length. Which gauge machine are you using? Perhaps we can find you a similar pattern in tuck stitch to guide you.
Thank you very much for your reply. I am not sure how to find the row gauge - I’m not that much of an advanced machine knitter and have only done sweaters in plain stockinette stitches and only know how to do a tension swatch in that particular stitch. The machine I am using is the Knitmaster/Empisal HK160, it’s a mid-gauge machine
An easy way to find your row gauge is to knit a few rows, knit a row in a contrast color, set your row counter back to zero, knit 50 rows, knit another contrast row , then a few more rows before taking it off of the machine. You have to let machine knitting “rest” a while (even overnight) before you measure it. Depending upon your project, you might even want to launder the piece before measuring. Now measure the knitting between the contrast rows. You will know how many rows are between the two contrast rows. The rest is a matter of doing a little math to determine how many rows you need to get the right length. Your swatch can be frogged and the yarn used in your project if you are concerned about running out of yarn.