Hola Emanuel,
You could replace it with "si" as long as you use the present indicative, not the present subjunctive.
However using "si" instead of "como" will make that nuance of warning/threat disappear somehow.
Como me ofrezcan el trabajo voy a hacer una gran fiesta.
If they offer me the job I will have a big party.
Si me ofrecen el trabajo voy a hacer una gran fiesta.
If they offer me the job I will have a big party.
Saludos
Inma