Is it permissible for a person with an excuse to perform ablution once a day and pray Dhuhr with it if it is difficult for him to perform ablution at his workplace, and is he sinful if he does so?

If performing ablution for every prayer is difficult, then there is no harm in performing two prayers with a single ablution, as long as no nullifier occurs. It is permissible to combine Dhuhr and Asr prayers to alleviate hardship. It is permissible for a woman experiencing irregular bleeding (mustahadah) and one not experiencing it to combine two prayers with a single ablution.