It’s a grey area of the law. In theory it’s possible, but once the subpoena has been delivered, the TRC can refuse to allow the possibility of signing a contract, as in fact they do in many cases. At the same time, the opposite thing it’s widespread: many volunteers over 27 (or over 25, today), have gone to the TRC and asked to be mobilized instead of signing a contract. To my knowledge, for example, the vast majority of the volunteers who joined the TDF in the early months of the invasion (a few hundred thousand men) are formally classified as mobilized. Even recently, several brigades used to suggest that a potential volunteer (above the minimum mobilization age) should go to the TRC, ask to get mobilized and then join that brigade through a letter of recommendation, instead of signing a contract.
In any case, the large increase in contracts signed after the approval of the new mobilization law is largely due to the fact, as reported by Ukrainian sources, that many people who wanted to sign up waited for the new law (which approval required quite a lot of months) in order to see which benefits it was bringing to new contract soldiers. In addition, the possibility of signing 1-year contracts for those under 25 was also recently introduced. Moreover, the new recruiting centers run by the UAF that are opening in Ukrainian cities are, reportedly, experiencing considerable success.
That said, since the beginning of this month, individual brigades can finally directly recruit volunteers without the need to go through TRCs, as was the case until now and something that frightened many people even from volunteering, due to the many problems TRCs may cause - with quite widespread cases of people who wanted to go to a given brigade being forced to go to others (only the intervention of the command of the brigade in which the men wanted to volunteer prevented that).