How is your particle effect set up?
You should be able to achieve this by adding a Initial velocity module to the particle effect and setting the distribution to Vector Uniform.
As an example: Screen capture - d864810bcc2e6dca7f3b46c18ba9e640 - Gyazo
Put max to 1000,1000,1000
and min to -1000,-1000,-1000.
This should give you the result you want.