Is it permissible to add to the four recommended rak'ahs before Asr, or to the two pre-Maghrib rak'ahs, with the intention of absolute supererogatory prayer? And is this legislated and recommended?

There is no impediment to performing more than four rak'ats before the 'Asr prayer, due to the permissibility of absolute supererogatory (nafl) prayers outside of times when prayer is prohibited. As for the two rak'ats before the Maghrib prayer, they are recommended (mustahabb) but are not among the regular supererogatory prayers (rawātib). It is not recommended to perform more than these two, as doing so would lead to delaying Maghrib beyond its prescribed time. Scholars have unanimously agreed that performing Maghrib prayer promptly at the beginning of its time is superior. The evidence has guided towards the recommendation of making them short, similar to the two rak'ats before Fajr. The wisdom behind their recommendation is the hope that supplications made between the adhan and iqamah will be answered.