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