NO you are not allowed to build time once with EK. What you come with is what you maintain with respect to accelerated commands. The whole accelerated command opportunity will likely not be of benefit to those who qualify, in the future. That is just my opinion, based on what has been told to me by those apparently 'in the know' regarding the latest uproar over mostly accelerated guys doing the transition course to the 777. No slag intended on the accelerated guys at all, just that this latest ammendated/revised/abused policy has caused another round of grief to every single airbus F/O and it has become quickly apparent to management.
Policy states that qualified F/O's will be first choice for upgrades, although i agree this has not always been the case. But as i see it, if you were to join today with the hopes of accelerated command it likely won't happen except in exceptional cases, as by the time you qualify there will be many non-accelerated F/O's who will have qualified in the next year and a half. This is not the case on the 777 at present, it is easily the case on the Airbus, but on both types will easily be the case within the next year to year and a half.