About 600 workers are set to take part in a strike over the plans on May 23 and 24.
Sean Gibson, of Unison, told the BBC staff are angry and do not want their employment to be shifted to an outsourced company.
Wrightington, Wigan and Leigh Foundation Trust said in a statement: “WWL Solutions Ltd is a wholly-owned subsidiary company of Wrightington, Wigan and Leigh NHS Foundation Trust, set up to manage the Trust’s Estates and Facilities and Procurement teams with the sole intention of continuing to deliver exemplary support services to the Trust and facilitating the best patient care.
“The creation of WWL Solutions Ltd enables the expansion of services to other public and private organisations.
“Numerous similar companies have been established within the NHS and have been proved to be both efficient and effective.”