Currently, you can't modify the stats on ships. There are some modded ships out there, such as Syn's Solitaire (YT-510 with HS 15). These were handed out as rewards some years ago by the administration.
For new modded ships, you will need to wait for R/D.
Customize, in terms of art, can be doing by hiring an artist and requesting a customize image for your ship. Once you have the image you like, then you can upload it for approval. After it is approved, then you can apply it to your ship (via the use of CPs).
The existing mods weren't admin rewards; they were all from red scenarios or various community games that are no longer done and will not return because they have the potential to be imbalancing and do not subscribe to any particular ruleset. Whether modding ships will be possible with R&D remains to be seen, but for the foreseeable future in any case, stat modifications cannot be applied to entities.