The jobdsl plugin documentation mentions that it is possible to use shared libraries:

https://github.com/jenkinsci/job-dsl-plugin/wiki/Real-World-Examples#import-other-files-ie-with-class-definitions-into-your-script

Despite of multiple searches I haven't found a way to use jobdsl with configured additional classpath. More over the configuration as code part brings some more constraints too. All references point to other mechanism which is a pipeline with a jobdsl step.

While it is said to do things in such a way it rather sounds like a workaround which multiply amount of jobs/configurations carried for basic scenarios. Effectively this approach double amount of configurations.

Hence my proposal is to introduce a new option for jobdsl CASC which would allow to configure classpath elements used for bootstrapped jobs and finally let them utilize advertised real word example:

jobs: 
  - additionalClasspath: ['/var/jenkins_home/lib/', 'abc/', 'def/']
  - script: >
      job('testJob1') {
          scm {
              git('git://github.com/quidryan/aws-sdk-test.git')
          }
          triggers {
              scm('H/15 * * * *')
          }
          steps {
              maven('-e clean test')
          }
      }

  - file: ./src/test/resources/javaposse/jobdsl/plugin/testjob.groovy
  - file: ./src/test/resources/javaposse/jobdsl/plugin/advancedJob.groovy