Welcome to gardening! Yes, you can add compost or good topsoil over the old soil, and it will still benefit the plant's roots. It just takes longer for the microorganisms (the tiny creatures in the good soil) to move to the plant roots and help them absorb nutrients. Many plants, however, do not like soil piled up very close to their stems. Damp soil touching a stem or trunk can cause rot. Take care not to bury the stems of a plant that is already growing. Keep growing, Marianne Binetti