You can open an existing non-Maven project and add a Maven support via IntelliJ IDEA UI.
Add Maven support
Open an existing project, for example, a Java project.
In the Project tool window, right-click your project and select Add Framework Support.
In the dialog that opens, select Maven from the options on the left and click OK. IntelliJ IDEA adds a default POM to the project and generates the standard Maven layout in Project tool window. IntelliJ IDEA also creates a corresponding structure with Lifecycle and Plugins in the Maven tool window.
Open the generated POM and specify a groupId. The artifactId and version are specified automatically. Every time you change the POM, IntelliJ IDEA displays a popup suggesting to import your changes.
At this point you can further develop your project using Maven. We recommend making all your project changes in POM since IntelliJ IDEA considers pom.xml as a single source of truth.
Create an executable JAR
You can conclude the following optional steps to create an executable JAR.
Click to build project. IntelliJ IDEA generates target folder. Note that IntelliJ IDEA only compiles sources and doesn't create either JAR file or Manifest file.
Create a Manifest file in the resources directory.
In your POM specify the Manifest file information, so you can use Maven to generate an executable jar file.
In the Maven tool window, in the Lifecycle list, double-click the install command to generate the jar file. IntelliJ IDEA generates an appropriate information in the target folder and an executable JAR in the Project tool window. You can right-click the generated JAR and select Run to execute the file.
If your existing project is larger and contains more than one module, converting such project into a Maven project becomes quite challenging. Since IntelliJ IDEA recognizes project settings only from POM when you convert your project you need to check and adjust the following settings:
Annotation settings - they are changed for the modules.
Compiler output - it is changed for the modules.
Resources settings - they are ignored and overwritten by POM.
Module dependencies - they need to be checked.
Language and Encoding settings - they are changed for modules.
Also, there is no POM template generated. All dependencies (including module dependencies) need to be manually included into POM.
In this case we recommend that you create an external POM where you describe your project and open your POM as you would open a regular Maven project using File | Open command.
IntelliJ IDEA adds POM to the project and a Maven layout for the existing elements. IntelliJ IDEA also generates the corresponding structure in the Maven tool window.
to build project. IntelliJ IDEA generates target folder. Note that IntelliJ IDEA only compiles sources and doesn't create either JAR file or Manifest file. 
