I have primer designed for conventional PCR and would like to use it for qPCR / RT PCR by adding probe as one reagent in master mix if that is possible.
It's going to depend on how you designed your primers.
If they are:
amplifying a very small (about 100 base pairs) product
located in exon regions, preferably spanning an intron
have 95-105% efficiency in qPCR
Then yes.
In general, primers designed for standard PCR don't meet the basic parameters for qPCR. Typically, the amplified product is too large and/or one or both primers are in an intron/promotor/other non-transcribed region.