Just in case people weren't aware, you can also force 64 bit node to build 32 bit packages by using the "arch" and "target_arch" flags on npm.

For example, part of my build looks like:

stage('Windows x86') {
    agent { label 'win-vs2017' }
    tools { nodejs 'NodeJS 8.9.4' }
    environment {
        npm_config_arch = "ia32"
        npm_config_target_arch = "ia32"
    }
    steps {
        bat "npm install"
        archiveArtifacts '*.zip, *.msi'
    }
}

Code changed in jenkins
User: Nikolas Falco
Path:
pom.xml
src/main/java/jenkins/plugins/nodejs/tools/CPU.java
src/main/java/jenkins/plugins/nodejs/tools/NodeJSInstaller.java
src/main/java/jenkins/plugins/nodejs/tools/Platform.java
src/main/java/jenkins/plugins/nodejs/tools/ToolsUtils.java
src/main/resources/jenkins/plugins/nodejs/Messages.properties
src/main/resources/jenkins/plugins/nodejs/Messages_it.properties
src/main/resources/jenkins/plugins/nodejs/tools/NodeJSInstaller/config.jelly
src/main/resources/jenkins/plugins/nodejs/tools/NodeJSInstaller/config.properties
src/test/java/jenkins/plugins/nodejs/tools/NodeJSInstallerTest.java
src/test/java/jenkins/plugins/nodejs/tools/ToolsUtilsTest.java
http://jenkins-ci.org/commit/nodejs-plugin/14e5121888d842e5b9679b2f7ae08ddce5cc1588
Log:
JENKINS-50378 Add an optional flag that allows to force the installation of a 32bit package of the NodeJS. The flag is allowed only in case a NodeJS package is available for the underlying system and 32bit architecture otherwise the build will fail.

We ship code that uses node.js on a desktop that uses native modules. On windows we need both 32-bit and 64-bit windows and we prefer using a single build machine that builds both the versions. We are currently using the manual approach where we specify the download URL to get the 32 bit version while it picks up the 64-bit version automatically.

It would be great if that could be an optional parameter(in the pipeline script). 32 bit versions can be compiled and prepared from 64 bit build machines.

Could you explain me when do you expect run 32 NodeJS?

Than expose in details how and where do you expect to mark/describe when the system should or must install 32 bit architecture and what's happen on system that does not support 32bit architecture.

This is not a bug, it works as design. Plugin get platform and its architecture using the properties given from java executable. If you want force a node to be executed to 32bit than run the agent with 32 JDK.