Hi Miroslava,
Try to use Textedit and maintain the read_only property as abap_true and mandatory property also abap_true. whenever user click on button, set read_only property as abap_false on button action.
So that it would appear with asterix mark.
If there is no button action, for textedit field and set the read_only and mandatory properties as ABAP_TRUE.
For your existing field, change the display type as "Text Edit" and set the above two properties.
Hope it will helps you.
Regards,
Naga