Fixing some endogenous variables to a constant during simulation

In this famous paper ( Kaplan, G., Moll, B., & Violante, G. L. (2018). Monetary policy according to HANK. American Economic Review, 108(3), 697-743.), the authors simulate a monetary policy shock…but in the budget constraint of the government (B_t + G_t + T_t = some \; tax \; revenues), they can, for example, fix B_t and G_t to a constant and allow only T_t to adjust after the policy shock.

So if you have in your model:
B + G + T = some taxes

Then you can do, for example, the following?
steady_state(B) + steady_state(G) + T = some taxes

That will work if the steady state of B and G can be found/determined.