Manchu is a typical low-resource agglutinative language, for which existing digital resources are largely limited to character recognition and basic annotation, while sentence-level generation remains underexplored. In this paper, we study constrained sentence-level Manchu continuation. We construct the first context-target corpus for this continuation task, consisting of 3214 examples collected from Manchu Stories in Translation, Common Manchu Dialogues, and other sources, covering both narrative and conversational genres. To improve robustness to rich suffixation, relatively flexible word order, and frequent OCR or transcription noise in Manchu, we propose a Multi-Granularity Data Augmentation (MGDA) strategy that expands training data through character-level noise injection, phrase rearrangement, and suffix perturbation, without relying on external dictionaries or translation models. Experiments on ByT5-small and Qwen2-1.5B show that MGDA substantially improves Manchu text generation performance: BLEU-4/ROUGE-L increases from 2.03/2.59 to 17.60/42.70 for ByT5, and from 1.98/6.42 to 14.12/41.49 for Qwen. Further ablation studies demonstrate the complementarity of the three augmentation granularities and suggest potential benefits of byte-level modeling in this experimental setting.