Yes, you can certainly do that. Whether or not it's advisable to do so, however,depends on your test purpose and how you intend to use the results.
Custom tests can be useful if they have a particular special purpose and if use of the results will be limited or controlled. But if called into questions, custom tests may not have the credibility of unmodified industry-standard approaches. Before configuring a custom test, it is recommended that you review the list of ISTA tests to see if there is a standard Proceudre or Project which would suit your purpose. Click here to review the full list of ISTA Proceudres and Projects.
In 2008, ISTA activated our 6-Series for Member Performance Tests. The 6-Series is dedicated to test protocols created by ISTA members to meet their particular purposes and applications. The tests may be completely original, or may be modifications or variations of ISTA Procedures or Projects or other publishes and accepted ests. The ISTA Testing Council reviews and approves these tests, but primary responsibility for creation, validation, maintenance revisions and support of 6-Series tests rests with the originating member. For further information on how to submit your company's protocol for approval as an ISTA 6-Series test, contact Meredith Dougherty (firstname.lastname@example.org).
Source: ISTA Staff