if $L \in NP$ then its mapping reducible to HALT?
It depends what type of reduction you're talking about. But if you mean a "many-one reduction with no time constraints" (a Turing machine that transforms instances of problem $L$ to instances of problem HALT such that the answers remain the same), then yes.

The algorithm goes something like this:


Take an input $x$
Use brute force to decide whether $x \in L$ (in potentially exponential but still finite time)
If the answer is "yes", let $T$ be a Turing machine that always accepts
If the answer is "no", let $T$ be a Turing machine that always diverges
Return $\langle T \rangle$


Now, when $\langle T \rangle$ is passed to a HALT oracle, the oracle will accept if $x \in L$, and reject if $x \not \in L$.