Absolutely. The option to create a custom test will appear in the "Create skills test page" AFTER you have created at least one test using the standard flow of using the templates and skills we have available - just to get you acquainted with the options available for you.
So once you have created your first test, click on "Create skills test" and choose the "Create your own test from scratch" at the bottom of the page.
Then use the "Add question" toolbar to start adding questions. Note you can create your own questions but also use our database questions by adding them in one-by-one.