Union Pacific Corp., which agreed two weeks ago to merge with Missouri Pacific Corp., yesterday announced plans to add the Western Pacific Railroad Co. to its growing system.

The merger proposal, worked out by the management of the two companies, will be submitted today to Union Pacific's executive committee and Western Pacific's board of directors for their approval. It calls for Union Pacific to make a cash tender offer of $20 a share for Western Pacific's common stock outstanding.

Purchase of all of Western Pacific's stock would cost UP about $25 million; it already owns 9.9 percent of Western Pacific's stock which it bought at the time of the company's public offering in early 1979.

The Western Pacific is a small railroad operating a total of 1,721 miles of track in California, Nevada and Utah. Its main line extends eastward from San Francisco-Oakland, joining the Union Pacific at Salt Lake City.

"It was a relatively small but logical extension of Union Pacific Corp.'s plan to strengthen the railroad, complementing the proposed Missouri Pacific merger by extending Union Pacific service to the San Francisco Bay area," James H. Evans, chairman of UP, said yesterday.

Assuming Interstate Commerce Commission approval of UP proposals to merge with both the Missouri Pacific and the Western Pacific, the resulting combination would create the third largest railroad system in the country with 22,900 miles of track.