There's an end of stupidity my dear friend. If you want to save space you can even move your other files to USB.
Nevertheless, if you're willing to do so. Here is the procedure.
1. Install Root explorer. (It's a paid app on play store)
2. Install all your apps as usual.
3. Through Root explorer, go to /data/apps folder and move all the *.apk you would like to have on your USB.
4. Then manually long click on each .apk and choose to create a link and link it to /data/apps/(here)
5. Take care every time you boot your phone you must have your USB connected otherwise the link would break and you'd have to relink them. Don't do this for Moor than 10 apps which have a size of at least 20MB.